My invention comprises improvements in an electronic control system of the kind described in U.S. Pat. Nos. 5,157,608, 5,081,886 and 5,029,087. It is an improvement also in a hydraulic control system of the kind shown in U.S. Pat. No. 4,665,770. Each of these patents is assigned to the assignee of my present invention.
Transmissions of the kind described in these prior art patents include multiple-ratio planetary gearing having clutch and brake structure for establishing multiple torque flow paths from an internal combustion engine to the traction wheels of an automotive vehicle. The multiple-ratio gearing defines a range of torque ratios for the torque flow paths. A hydrokinetic torque converter situated between the engine and the torque input element of the gearing comprises an impeller connected to the engine and a turbine adapted to be drivably connected to torque input elements of the gearing. A friction bypass clutch that forms a part of the torque converter is adapted to establish a positive driving connection between the impeller and the turbine when the torque converter achieves a hydrokinetic coupling condition. The bypass clutch unlocks to permit normal open converter operation when the vehicle driving conditions demand hydrokinetic torque multiplication.
The transmission includes a positive displacement pump that forms a part of a control valve system. Friction clutches and brakes control relative motion of the elements of the gearing and establish and disestablish torque reaction points in the gearing as the multiple ratios are effected. Fluid pressure-operated servos actuate the clutches and brakes.
A main regulator valve in the valve system maintains a circuit pressure as the valve system establishes controlled distribution of pressurized fluid to a transmission lubrication fluid circuit as well as to the hydraulic torque converter.
An electronically controlled throttle pressure solenoid establishes a solenoid feed pressure for various shift solenoids that are under the control of a microprocessor. The shift solenoids, in turn, are in communication with shift valves that form a part of the valve system. The shift valves selectively control distribution of actuating pressure to the clutch and brake servos. The solenoid feed pressure is distributed to the main regulator valve which effects the distribution of a control pressure to a bypass clutch control valve and to the lubrication fluid circuit. The bypass clutch control valve responds to operating variables either to engage or to disengage the bypass clutch.
If the operating conditions are such that a high pump output is required to accommodate a transient operating condition (e.g., a rapid change in ratio), the main regulator valve will respond by interrupting distribution of pressure to the torque converter and to the lubrication circuit until the valve system stabilizes and the demand for pressurized fluid by the valve system subsides.
It has been observed that there is a tendency for the bypass clutch and the torque converter to lose pressure during such transient condition when the supply of fluid to the converter is interrupted. This is due to the tendency of the fluid passages communicating with the converter to bleed down. When the transient condition subsides and pressurized fluid is reintroduced to the converter circuit, an uncontrolled harsh engagement of the bypass clutch may occur if the bypass clutch control valve is positioned to cause the bypass clutch to engage. Engagement timing of the bypass clutch, furthermore, may vary from its calibrated value due to a temporary reduction in converter pressure that results from such a transient condition.